The drive towards increased areal density and higher signal to noise ratio in magnetic recording applications has necessitated the use of magnetoresistive (MR) sensors. Magnetoresistive (MR) sensors or heads are very attractive for magnetic recording applications because they have been shown to be capable of reading data from a magnetic medium at great linear densities. A MR sensor detects magnetic field signals through the resistance changes of a read element made from magnetoresistive material as a function of the magnetic flux being sensed.
The prior art teaches that for optimal performance an MR head requires two bias fields. A transverse bias field, which orients the magnetization in the plane of the MR element at approximately 45.degree. relative to the direction of the sense current, is provided so that the response of the MR head to magnetic flux is linear. A longitudinal bias field, which extends parallel to the surface of the magnetic media and parallel to the lengthwise direction of the MR element, is provided to suppress Barkhausen noise by maintaining single domain states in the end regions of the MR head. Many different means have been employed both to linearize the sensor output and to provide for a single domain in the sense region. A number of patents have issued in this area. Exemplary of these are U.S. Pat. Nos. 4,663,685, 4,967,298, 5,079,035 and 5,005,096, which disclose MR heads using different schemes for transverse and longitudinal biasing.
However, the drive towards improved recording densities has led to the requirement for narrower recording tracks and increased linear recording density along the tracks on magnetic storage media. The use of narrower tracks makes it very critical that a MR head be positioned accurately over the track to be read as it gets repositioned during seek from one track to another by the associated servo-system. However, prior art MR heads exhibit asymmetric off-track performance that makes it difficult to position a MR head accurately over narrow tracks during seeks, thereby severely challenging the associated servo-system.
Also, MR heads are read only heads and hence require the use of separate write elements to write data to disk. Often the write operation results in zig-zag domains being formed at the edge of a track resulting in poor performance of magnetic recording systems using MR read sensors. Therefore, it is desirable for the MR sensor to be narrower than the write head so that the MR sensor is always positioned within the outer bounds of the written track. Using a narrower MR sensor results in the associated servo mechanism being further challenged because it restricts the operating range of the servo system.
Head positioning servo-mechanisms are used to accurately locate a MR head over a track on the magnetic media to maximize head performance. The servo-system has two primary functions: (1) to determine the position of the head relative to the desired track and (2) to keep the position of the head on-track by generating a position error signal by comparing the actual head position against the desired head position. The operating range of the servo-system is defined as the range of position offset values over which the relationship between the position offset of the MR head and the position error signal is linear. The operating range for the servo system is maximized and hence system performance is maximized when the slopes of the sides of the off-track profile of the MR head are equal, i.e., when the off-track performance profile of the MR head is symmetric.
Due to anisotropic flux propagation MR heads exhibit asymmetric off-track performance. This results in the location of the physical center of a MR head to be different from the location of its magnetic center. Generally, the servo-system is setup, using servo bursts that are written on disk, to accurately locate the physical center of the MR read head on the desired track. However, it is the magnetic center of the MR head that needs to be centered on the track to maximize the signal to noise ratio during operation. Hence, measurements of the offset between the location of the physical center and the magnetic center of a MR head are essential in order to write servo bursts to accurately position the magnetic center of the head on the desired track.
However, the location of the magnetic center of a MR head is dependent on the stripe height, width and thickness of the MR element in the head. Variations in MR head manufacturing processes lead to variations in the location of the magnetic center and makes it difficult to accurately predict its location. By being able to minimize the variation in location of the physical center and the magnetic center of a MR head, process variability concerns are minimized, while the need for offset measurements between the location of the physical center and the magnetic center of a MR head are also eliminated. Making the off-track performance of an MR head more symmetrical results in minimizing the variation in location of the physical center and the magnetic center of a MR head thereby improving the operating range of the servo-system, while making it possible to achieve higher track densities on the magnetic media.
It is possible to reduce the asymmetry in the off-track performance profile of a MR head by reducing the stripe height. However, reducing the stripe height significantly degrades the signal to noise ratio of the MR head system, especially for drives requiring constant current density through MR heads during operation. Further, smaller stripe heights require tighter process tolerances and thereby further constraining the MR head manufacturing process.
Hence, there is a need to make the off-track performance of a MR head more symmetrical and to minimize the variation between the magnetic center and the physical center of a MR head.